The present invention is directed to a digital servo system, and more particularly, a digital servo system for use in electro-mechanical systems requiring the rapid movement and precise position of a movable member. Such systems may include impact printers (including daisywheel impact printers), magnetic disk memories, and so forth. The present invention is described with reference to controlling the carriage of a printer but may also be used to control the movement of the printwheel of a daisywheel printer, the movement of a magnetic head in a disk memory and in other applications.
In the prior art, servo control systems employing predominately analog circuitry have been used to control the printwheel and carriage of a daisywheel printer and the arm mounting the magnetic pickup of a disk memory unit. For example, U.S. Pat. No. 3,696,354 teaches an analog servo control system for controlling the arm of a magnetic memory and U.S. Pat. Nos. 3,789,971 and 3,954,163 described the application of the aforementioned servo control system to printers having rotatable elements.
Of course, those skilled in the art will recognize that to accomplish high quality printing or to accurately position the arm of the magnetic disk memory, the servo signals of the servo system must be accurately controlled. In such servo systems, an encoder wheel, either electro-optically or electromagnetically, generates a pair of phase spaced cyclic analog position signals which are representative of the phase position of the movable member of the printer or magnetic memory. Each cycle in the analog position signal identifies a small increment of movement. As explained in the aforementioned patents, differentiation of such signals indicates the velocity of the movable member, which velocity is compared with a command velocity a closed loop servo control system during a coarse mode of operation. During a subsequent fine mode of operation, the differentiated position signal and cyclic position signal are feedback in the closed loop servo control system to electronically detent the movable member. In these prior art systems, the position and velocity signals are both amplitude and phase sensitive. To help compensate for such sensitivity, a floating reference threshhold signal was sometimes utilized in the generation of the command signal, as is described in U.S. Pat. No. 3,954,163. In the prior art, both the compensation and the generation of these servo signals was done using analog techniques. Of course, those skilled in the art will recognize that analog circuit techniques suffer from drift and therefore are prone to become misadjusted with the passage of time. Of course, digital circuit techniques are much less sensitive to drift and other factors tending to degrade system performance.
It was therefore one object of the present invention to provide an improved servo control system for controlling the movement of the printwheel and/or carriage of an impact printer or the arm of a magnetic memory.
It is yet another object of the present invention to provide a servo system having more digital circuitry than prior systems thereby reducing the need for analog compensation.
The foregoing objects are achieved as is now described. The digital servo system control the motion of a movable member along a path from a starting position to a destination position. The member is moved by a motor and is coupled to an encoder for generating at least two phase spaced cyclic analog position signals representative of the instantaneous position of the movable member. The periods of these signals is indicative of the instantaneous velocity of the movable member. The two cyclic signals are digitized to provide digital signals which change state as the corresponding signal changes polarity. At least one of the cyclic signals is inverted. Comparisons are made between one of the cyclic signals and the inverted and non-inverted forms of the other cyclic signal in two comparators. These comparators provide additional digital signals which change state as the inputs become relatively more positive and/or negative with respect to each other. The outputs of the two digitizers are exclusively orred in a first exclusive OR circuit while the output of the two comparators are exclusively orred in a second exclusive OR circuit. The outputs of the two exclusive OR circuits are in turn exclusively orred providing a pulse train which changes state in time with the analog position signals. In fact, in the embodiment described, the pulse train changes state eight times through one complete cycle of one of the analog position signals and will accurately change state during each 45 degrees of phase change in such analog signal. The pulse train is preferably applied to a one-shot which is responsive to change of state in the pulse train for outputting a short pulse in response thereto. The output of the one-shot is a pulse train of constant width pulses whose leading edges occur at each 45 degrees of phase change in the analog position signal. The pulse train is coupled via a constant current source to an integrater circuit which provides an accurate velocity signal during coarse mode operation of the servo mechanism. The velocity signal derived from the pulse train is compared with a command velocity in an error generator, the output of which is used to drive the motor which moves the movable member. If a coarse-fine dual mode servo system is utilized, then the velocity comparision is made during the coarse (velocity) mode while the zero crossing of the analog position signal is used to electronically detent the movable member in the fine (position) mode.